Exam Results
30
25
Mean: 76
Median: 79
20
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5
0
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10
15
20
25
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35
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Exam Results: Grading Policies
• Our grading policies as outlined on the website
are as follows:
• Approximately half of the class will get A's & B's;
the other half C's & below.
– This means if you were above a 79, that is an A or B on
the exam
• Students getting 50% or more of a perfect score
on their total score of exams, concept quizzes,
and HWs will get at least a C.
Chapter 7: Processing the Image
• The Human Visual
System
• Lightness Perception
– Brightness
– Lightness constancy
• Lateral Inhibition
– Edge enhancement
– Lightness contrast
The Human Visual System
• The photoreceptors turn light into electrical
signals
• The nerve cells in the retina do some low level
image processing
• Then transmit the signal to the optic nerve and
the brain
Light
Structure of the Retina
← Nerve cells
← Photoreceptors
← Choroid
Brightness vs. Lightness
• In this course, brightness and lightness do not
mean the same thing!
• Brightness is the overall light level of a scene, and
is related to the intensity of the light source
• It also depends on the degree of dark adaption
• The opposite of “bright” is “dim”
Lightness
• Lightness is a property of a material surface and
how much light it reflects
• It is independent of illumination and dark
adaptation
• The opposite of “light” is “dark”
Lightness Constancy
• Because our perception of light is independent of
illumination and dark adaptation, it must depend
on the relative lightness of objects compared to
their surroundings.
• A white object in the shade may have less light
coming from it than a dark object in bright
sunlight, but we perceive them as light and dark
because of lightness constancy
Lightness Constancy: Comparison
•
•
The one below is seen in less
illumination than the one at right
The relative darkness and lightness of
different parts of the image is preserved
when there is more illumination
•
The lightness of the part of the dog in
the white rectangle at left is actually
almost the same as the part of the dog
in the white rectangle below!
Concept Question: Lightness
• You are sitting outside reading a book and the sun
goes behind a cloud. What happens to the
brightness and lightness of the white background
of the book pages?
A.
B.
C.
D.
They both stay the same
Brightness stays the same, lightness changes
Lightness stays the same, brightness changes
They both change
Lateral Inhibition
• How does our visual system
determine relative light
intensity?
• Recall that the neural
network in the retina is
connected laterally.
• This allows the signal at one
point on the retina to affect
the signal at other points on
the retina
Lateral Inhibition
• Increased illumination in one region of the retina
diminishes the signal to the brain from a
neighboring region
• This makes the total signal sent to the brain
relatively insensitive to overall illumination
changes, and very sensitive to differences in
illumination
Lateral Inhibition
• A signal received at A will
send a positive (excitatory)
signal to its own neural cell
and a negative (inhibitory)
signal to B’s neural cell.
A
• A signal at B will do the
reverse.
to the brain
B
Lateral Inhibition
• If the overall light level
increases, the signals from
each receptor increase,
but so does their ability to
inhibit one another.
• The net result is that the
change in illumination
does not change the total
signal to the brain very
much.
A
to the brain
B
Edge Enhancement
• Imagine that only A is
illuminated and there are
no lateral neural
connections.
A
• The neural cell at A sends
a signal corresponding to
the light level, and the
neural cell at B sends
nothing.
to the brain
B
Edge Enhancement
• Now we add the lateral neural
connections.
• Now the neural cell at B is
inhibited by the signal from A,
but B does not inhibit A
because it is dark.
• This gives a smaller signal at B
than if there were no lateral
connections, so the difference
between A and B is larger. to the brain
A
B
Lateral Inhibition
Each ganglion cell (the top
rung of the neural cell
system) is connected
through other neural cells
to a region of
photoreceptors that is
called its “receptive field”
The ganglion cell receives
signals from all of these
photoreceptors
Receptive Field: No Light
The ganglion cell produces some background response even
when there is no light on its receptive field
Ganglion cell
Intermediate neural cells
Photoreceptors:
Pink is excitatory
Green is inhibitory
Receptive Field: Center Illumination
When the center photoreceptors are illuminated, the ganglion
cell response is large
Ganglion cell
Intermediate neural cells
Photoreceptors:
Pink is excitatory
Green is inhibitory
Receptive Field: No Light
When light hits the surround receptors, the response of the
ganglion cell is inhibited.
Ganglion cell
Intermediate neural cells
Photoreceptors:
Pink is excitatory
Green is inhibitory
Receptive Field: No Light
If light hits the entire receptive field, the excitation from the
center receptors and the inhibition from the surround receptors
cancels out, and the ganglion cell response as it does to no light
at all
Ganglion cell
Intermediate neural cells
Photoreceptors:
Pink is excitatory
Green is inhibitory
Determining Response
The inner area (excitatory) can be
thought of as plus 1
The outer area (inhibitory) can be
thought of as minus 1
-1
+1
If the illumination is equal on
both areas, the effect is +1-1=0
If the center is bright and only
half of the outside is bright, the
you have +1-1/2=+1/2, so you
have a larger response.
Summary of Lateral Inhibition
Concept Questions: Lateral Inhibition
•
Shown below are 6 different centersurround receptive fields in the image
on your retina as you look at a dark
grey shape on a white
background. The receptive fields are
labeled A, B, C, D, E, and F. Each
receptive field is connected to a
different (ganglion) nerve cell.
Bright light
on white region
• Which receptive fields neither
enhance nor inhibit their ganglion
cell's signal?
a) F, b) B, d) D, e) F, B and D
• Which receptive field inhibits its
ganglion cell to send the weakest
signal (indicating the darkest
region)?
a) A, b) B, c) C, d) D. e) E
• Which receptive field stimulates its
ganglion cell to send the strongest
signal (indicating the lightest
region)?
a) A, b) B, c) C, d) D. e) E
Little light on grey region
The Craik-O’Brien Illusion
Edge Illusions in Art
• The change in reflectivity between the lightest
and darkest parts of a painting or photograph is
only 10-15 times, while the difference in
brightness in a real scene may be 1,000 to
1,000,000 times.
• Artists compensate for this using this same type
of edge illusion
From Seurat’s “Bathers at Asnières”
Water gets lighter
right before edge
Skin gets darker
right before edge
Seurat’s “Poseur en profil”
Darker near
the edge